Abstract

Although Cenozoic crustal shortening and thickening by thrust faulting have built the present Tien Shan, active right-lateral shear on the northwest-trending Talas-Ferghana fault appears to be the most rapid localized deformation in the belt. Ephemeral stream valleys have been offset right-laterally tens of metres. New and published radiocarbon dates of organic material deposited in depressions blocked by offset ridges place upper bounds on the average Holocene slip rate at 18 localities. Uncertainties allow 14 upper bounds to overlap the range of 8–16 mm/yr, and 95% confidence limits on such bounds at 11 sites are entirely within this range. We infer that the rate of ≈10 mm/yr is not simply an upper bound, but applies to the late Holocene Epoch. Although the bounds on rates permit more rapid slip in the northwest than the southeast, they do not place a useful constraint on variations in slip rate along the fault. Offsets of Paleozoic facies boundaries imply a total right-lateral shear of 180–250 km, but Early Cretaceous sedimentary rock appears to have been offset only 60 ± 10 km. Published paleomagnetic declinations of Cretaceous- Miocene rock demonstrate 20°–30° of counterclockwise rotation of the Ferghana Valley, which lies just west of the Talas-Ferghana fault, with respect to stable parts of Eurasia and 20° ± 11° with respect to the central Tien Shan east of the fault. These declinations are consistent with a maximum northwestward translation of 70–210 km of the Ferghana Valley at the Talas-Ferghana fault and, therefore, with a similar maximum horizontal shortening across the Chatkal Ranges, which lie between the Ferghana Valley and the Kazakh platform. Estimates of crustal thickness beneath the Chatkal Ranges, however, permit only 60–100 km of Cenozoic shortening. If <100 km of slip on the Talas-Ferghana fault accumulated at a constant rate of 10 mm/yr, it would imply an initiation of slip more recently than ca. 10 Ma, long after India collided with the rest of Eurasia.